Many manufacturers, such as Hitachi Metals and Vacuumschmelze sell amorphous alloy ribbon, which is precursor to nanocrystalline alloy, with a maximum width up to 63.5 mm. The current maximum width is limited by the casting technology, which results in poor magnetic properties, large thickness variations across the width of the ribbon and poor winding capability during casting.
There is significant demand for nanocrystalline foil alloys used in power electronic devices. The low loss properties for nanocrystalline ribbon make them suitable for a wide range of high frequency (kHz) transformer applications. The nanocrystalline ribbon is also used in choke coils to reduce high frequency harmonics. The nanocrystalline ribbon can also be used in pulsed power applications.
The nanocrystalline alloys are produced through a planar flow casting process where molten metal is fed onto a rotating quench wheel where the metal is rapidly cooled into an amorphous state at cooling rates on the order of 106° C./sec. The preferred thickness for the as-cast ribbon is between 13 and 20 microns. The linear speeds of the rotation quench wheel are typically between 25 and 35 m/s. The ribbon is cast continuously and stripped from the quench wheel and mechanically conveyed onto a large spool moving at the same speed where it is continuously wound.
Conventional iron-based fully amorphous alloys are commonly used in transformer cores, and the ribbon is available at widths of 5.6″, 6.7″ and 8.4″ at a thickness of 25 microns. This nanocrystalline alloy being of only 13 to 20 microns in thickness makes catching and winding the ribbon very difficult at widths beyond 63.5 mm. The relative thinness of the ribbon makes it difficult to mechanically catch the ribbon at high speeds without breaking it, and, therefore, the ribbon cannot be wound continuously onto a spool.
The thickness uniformity in the width direction also limits the ability to continuously wind the ribbon onto a spool. Thickness variations can cause the spool to wind poorly as the spool builds due to high and low sections of the ribbon progressively overlapping. For example, a spool consisting of ribbon with large thickness variation across the width will be very loose where the ribbon is thinner and very tight where the ribbon is thicker causing the ribbon to easily break during winding.
The difficulty in continuously winding the ribbon is one of the reasons that wider nanocrystalline alloys are not commercially available. While it is possible to cast the ribbon and wind onto a spool in two distinct stages, this is difficult as a practical matter because it introduces many folds and wrinkles into the ribbon that can detract from the soft magnetic performance. Continuous casting and synchronous winding of the ribbon is also need to reduce the cost to produce the ribbon because it eliminates the intermediate processing steps.
The fully amorphous ribbon is then heat treated into a nanocrystalline state. U.S. Pat. No. 4,881,989 entitled “Fe-base soft magnetic alloy and method of producing same”, the contents of which are incorporated by reference, discloses the physics of the transition from amorphous as-cast ribbon into a nanocrystalline alloy during heat treatment.
The narrow available width limits the applications to mainly small tape wound core materials. Producing a wide high frequency transformer currently requires stacking multiple narrow wound cores together. The narrow ribbon width also limits the production rates of the nanocrystalline ribbon which keeps the cost of the ribbon prohibitively high for many applications. The thickness of the foil being less than 20 microns makes winding ribbons greater than 63.5 mm difficult, and such wider ribbon is not commercially available.